1. Field of the Invention
The invention relates to a process for removing HCN and NH.sub.3 from a gas, such as a synthesis gas, to produce a clean gas containing less than 20 vppb of a combined total of HCN and NH.sub.3, via catalytic hydrolysis, water scrubbing and adsorption, and to a hydrocarbon synthesis process using the clean synthesis gas.
2. Background of the Invention
The removal of HCN and NH.sub.3 from gas streams is sometimes required to prevent catalyst poisoning, product contamination and for environmental reasons. For example, Fischer-Tropsch type catalysts useful for forming hydrocarbons from a synthesis gas mixture of H.sub.2 and CO (syngas) are known to be deactivated by HCN and NH.sub.3. Producing syngas from a fossil fuel which contains nitrogen or nitrogen compounds also forms NH.sub.3 and HCN, which can deactivate these and other catalysts. Methods for reducing the HCN content of various gasses, including syngas, have included catalytic hydrolysis (U.S. Pat. No. 4,769,224) and chemical scrubbing (U.S. Pat. No. 5,068,254). However, while these patents disclose reducing the HCN level down to about 0.1 ppm or 100 ppb, it has now been found that even as little as 100 vppb of a combined total of HCN and NH.sub.3 in the syngas results in unacceptably short Fischer-Tropsch catalyst life. It has also been found that deactivation of such catalysts by HCN and NH.sub.3 may be reversed and catalytic activity restored by contacting the deactivated catalyst with hydrogen. Consequently, the catalyst must be at least periodically rejuvenated with hydrogen to restore its activity. In a fixed bed reactor this means taking the reactor off-line. In a fluid bed reactor the reactor is taken off-line or the catalyst particles are circulated between the reactor and a rejuvenation vessel. In a slurry process the reactor is taken off-line or the catalyst may be rejuvenated either intermittently or continuously while the reactor is on-line. Up until the present invention, it has not been practicably feasible to reduce the level of the HCN and NH.sub.3 catalyst poisons in the syngas to where the catalyst has a commercially acceptable half life requiring minimal rejuvenation.